Tanel Tätte (PhD), University of Tartu
Uno Mäeorg (PhD), University of Tartu
Ilmar Kink (PhD), University of Tartu
Alex Rozhin (PhD), School of Engineering and Applied Science, Aston University, UK
Mihkel Koel (PhD), Department of Chemistry, Faculty of Science, Tallinn University of Technology, Estonia
Integration of two naturally very different classes of materials to bring out their best properties, while "cancelling" their worst ones, to make a better material is one of the main challenges of modern material science. The thesis deals with the synthesis and characterization of composite materials based on metal oxides and carbon nanotubes. The goal of this dissertation was to study formation mechanisms of oxide nano- and microstructures from metal alkoxides by sol-gel methods, and to search for ways to increase their electrical properties by doping with carbon nanotubes.
Viscosity and rheological properties of metal alkoxide sols are the main parameters responsible for formation of the specific shape of oxide structures via sol-gel processes. Sol precursor's viscosity in turn is affected by the sol particle size, in this study evaluated to be approximately 4 nm in length and 2 nm in diameter. Multiple metal alkoxides polymerization-condensation processes and the corresponding materials properties were investigated and characterized. The best spinnability conditions, i.e. material ability to form liquid thread, were also determined for different metal alkoxides. Rheological tests proved that the metal alkoxide precursors behave as typical non-Newtonian fluids. Rheological studies of precursors doped with carbon nanotubes demonstrated unusual decreasing of viscosity.
During this Ph. D. study, preparation of structures with different shapes was demonstrated. Metal oxide fibers with a high aspect ratio of up to 10 000 and as small as 200 nm were drawn from different concentrated metal alkoxide precursors. The pinching-off of alkoxide based jets in air was demonstrated as a reproducible route for preparing nanometer level sharp oxide needles. Also, it was also shown that the modified tape casting can be used to prepare either smooth and/or linearly structured sol-gel films from high-viscosity sol.
Using pristine CNTs and metal alkoxides, it was demonstrated that pulling concentrated alkoxide/CNT viscous threads orients the nanotubes inside the matrix. At 1% CNT loadings, the final CNT reinforced CMC material electrical conductivity was measured to be in the range of 10 S/m. Demonstrated method is applicable to obtain a variety of shapes from CNT reinforced CMCs with different metal oxide matrices.